Wiki article on MCB/cable size

[JohnW2]

I've just had a quick look through this site's wiki, so as to avoid wasting people's time by asking questions which are already answered there.

I am a bit confused by the article about the relationship between MCB size and cable size (in the "Fuses and MCBs" section). Unless I'm missing something, it looks to me as if it is either incorrect (and potentially dangerous) or, at least 'worrying'. The article contains the statement:

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:

Is this not 'back to front'? i.e. should it not be "the current carrying capacity Iz of the cable must be no less than I2 (i.e. 1.45 times In) ......."

I certainly would not be very comfortable with having a cable carry 1.45 times its current carrying capacity for up to one hour whilst waiting for the MCB to operate. If the article does mean what it says, it would be simpler to just say "Iz must be no less than In" (since I2=1.45*In) - but I do find it hard to believe that this was the intention. Indeed, such a statement would seem to be contrary to the whole point of the article - which I thought was to indicate that (since I2>In] a cable just adequate for the rated current of the MCB (In) (which might sound OK to a non-electrician person) is actually not sufficient.

...or am I missing something?

Kind Regards, John

 

[ban-all-sheds]

I've just had a quick look through this site's wiki, so as to avoid wasting people's time by asking questions which are already answered there.

Thanks for doing that, and thanks for the feedback (as it was me wot writ the Wiki article).

I am a bit confused by the article about the relationship between MCB size and cable size (in the "Fuses and MCBs" section). Unless I'm missing something, it looks to me as if it is either incorrect (and potentially dangerous) or, at least 'worrying'.

It is not incorrect.

Is this not 'back to front'? i.e. should it not be "the current carrying capacity Iz of the cable must be no less than I2 (i.e. 1.45 times In) ......."

It can't be expressed that way because I2 is only 1.45In for Type B MCBs. For other devices it's different, e.g. for rewirable fuses it's 2In.

I certainly would not be very comfortable with having a cable carry 1.45 times its current carrying capacity for up to one hour whilst waiting for the MCB to operate.

Well that's what can happen. It's a common misconception that an X amp device will operate as soon as the current exceeds X amps, but this is not the case. A 32A MCB will pass 36A for ever. A 30A rewirable fuse will pass 54A for ever.

This link will lead to to lots of examples of time-current curves which show how quickly different types of devices operate at different currents.

If the article does mean what it says, it would be simpler to just say "Iz must be no less than In"

It does say that in the line immediately before the "Finally..." one you quoted.

(since I2=1.45*In)

See above re that.

but I do find it hard to believe that this was the intention. Indeed, such a statement would seem to be contrary to the whole point of the article - which I thought was to indicate that (since I2>In] a cable just adequate for the rated current of the MCB (In) (which might sound OK to a non-electrician person) is actually not sufficient.

All devices have an I2 > In to some extent. Where he device is a rewirable fuse you have to multiply the tabulated value of the cable's Iz by 0.725 to allow for the fact that the ratio there is 2, not 1.45.

But, of course 0.725 = 1.45/2, i.e. 1.45 ÷ the I2:Iz ratio.

When that ratio is 1 (as it is for Type B breakers) no adjustment is necessary, but for any other device you should do a second check, in addition to Ib ≤ In ≤ Iz for 1.45Iz≥I2 when choosing a cable.

...or am I missing something?

If you look at the diagram with the new info that I2:Iz is not always 1.45 hopefully it will be clearer.


Thanks for the feedback - it's very useful - I'll look at making the Wiki article clearer.

 

[JohnW2]

Thanks, and you're welcome. I understand everything you're saying, but it really doesn't make much sense to me in terms of common sense and safety (hence what I would expect regulations to require) ....

I am a bit confused by the article about the relationship between MCB size and cable size (in the "Fuses and MCBs" section). Unless I'm missing something, it looks to me as if it is either incorrect (and potentially dangerous) or, at least 'worrying'.

It is not incorrect.

Is this not 'back to front'? i.e. should it not be "the current carrying capacity Iz of the cable must be no less than I2 (i.e. 1.45 times In) ......."

It can't be expressed that way because I2 is only 1.45In for Type B MCBs. For other devices it's different, e.g. for rewirable fuses it's 2In.

As the intro to the article makes very clear (in bold), the calculations presented relate to the example of a Type B MCB and you have therefore correctly brought the 1.45 factor into play throughout the explanation. My concern is that, from considerations of safety (that's surely what current carrying capacity of cables is all about?) one should be dividing, not multiplying, the cable's current carrying capacity by the factor (I2/In), whatever it may be (1.45, 2.0 etc.) for the device concerned (if one wanted to compare the cable's capacity with In) - or, equivalently, multiplying the device's In by that factor in order to determine the cable carrying capacity required.

I certainly would not be very comfortable with having a cable carry 1.45 times its current carrying capacity for up to one hour whilst waiting for the MCB to operate.

Well that's what can happen. It's a common misconception that an X amp device will operate as soon as the current exceeds X amps, but this is not the case. A 32A MCB will pass 36A for ever. A 30A rewirable fuse will pass 54A for ever.

Exactly, and that's why I would have assumed that, in the interests of safety, one should use a cable whose capacity is certainly adequate to carry the current that the device could pass 'for ever' and probably also the current which (in the case of an MCB) the device could allow to pass for up to 1 hour? This article is stating (in two different ways) that it is adequate for the cable's tabulated current carrying capacity to only be adeqaute for In, but not for I1 or I2. Whilst I realise that there is undoubtedly a 'safety margin' built into the tabulated 'current carrying capacities' of cables, such a practice would seem to make a mockery of having such tabulated values which we are expected to work to. I don't think I've seen (nor would expect to see) anything to suggest that it's OK to exceed tabulated current carrying capacities by 13% indefinitely and 45% for up to an hour - but maybe I've missed that.

If the article does mean what it says, it would be simpler to just say "Iz must be no less than In"

It does say that in the line immediately before the "Finally..." one you quoted.

(since I2=1.45*In)

See above re that.

Indeed, but as I've said, these are just two statements of the same thing (both of which are saying that it's OK to use a cable not rated for I1 or I2) - since you are using the example factor (I2/In) of 1.45, then 1.45Iz>I2 is mathematically exactly the same as In<Iz. The same would be true if you were talking about a device for which the I2/In factor was something other than 1.45. If you call that factor f, then fIz>I2 is always going to be the same as In<Iz.

All devices have an I2 > In to some extent. Where he device is a rewirable fuse you have to multiply the tabulated value of the cable's Iz by 0.725 to allow for the fact that the ratio there is 2, not 1.45.
But, of course 0.725 = 1.45/2, i.e. 1.45 ÷ the I2:Iz ratio.

Albeit somewhat of a side issue, this is now getting very confusing. Multipying the cable's tabulated Iz by a factor less than 1 (e.g. 0.725) is, indeed, going in the correct direction for safety - multiplying by 0.725 is the same as assuming an I2/In ratio of 1.38, if one is going to then compare adjusted capacity with In. However, if you are saying that the I2/In ratio for a rewirable fuse is 2.0, then I would have thought that one surely should be dividing the cable's tabulated Iz by 2 (rather than 1.38 ) if one is going to compare it with In to determine whether the cable is adequate to carry I2. However, from what you go on to say, it sounds as if the 2.0 ratio may be something more confusing/messy than simply I2/In for a rewirable fuse.

As for "0.725 = 1.45/2, i.e. 1.45 ÷ the I2:Iz ratio", I find this even more confusing. As you've said, the 1.45 figure is specific to Type B MCBs - so are you saying that all other protective devices are referenced by a 'correction factor/ratio' to Type B MCBs. Whatever, there has surely got to be something wrong with dividing an adjustment factor of 1.45 (an increase of 45%) by 2 and thereby turing it into an adjustment factor of 0.725 (a decrease of 27.5%) would appear rather bizarre. There is surely no sense in the implication that a cable needs 'de-rating' (if one is going to size it in relation to In) in the case of a rewirable fuse (which makes sense) but can be 'up-rated' for an MCB (which does not make sense)!!

This really does seem all wrong to me. I would have expected that the regulations would be simple, merely requiring that cables should be sized so as to have an adequate current carrying capacity to carry that current which the protective device colud allow to flow for appreciable periods of time (i.e. certainly I1 and probably I2) - wheras this article not only makes it seem more complicated than that but seems to be suggesting that it is OK to use a cable whose current carrying capacity is theoretically not adequate for I1. let alone I2). Am I wrong?

Kind Regards, John

 

[skenk]

The Current Carrying Capacity of the cable isn't it's true current carrying capacity

 

[EFLImpudence]

Can I ask a question or two and put forward a theory because, whilst I agree with JohnW2, I don't know the answer?

Example - B32 mcb.
Would I be correct in assuming that the I2 figure (46.4A) is to allow the mcb to 'run cool' in normal service. If the mcb were designed to trip at 33A then the bi-metal strip which operates it would, presumably, be hot all the time and constantly on the verge of tripping the mechanism. Thus leading to a much shorter life.

Can we also deduce that the conductor is actually capable of carrying the current of the I2 value safely and the regulations are merely very conservative in the figures we have to use? What current can a 2.5mm² conductor sustain without melting?

If the cable had to be rated for the I2 value then RFCs would have to be of 4mm² and Radials of 6mm²(just). Any derating would lead to huge cables everywhere.

If I have it wrong then what is the reasoning and is JohnW2 correct?

 

[JohnW2]

I've just had a quick look through this site's wiki, so as to avoid wasting people's time by asking questions which are already answered there.

Thanks for doing that, and thanks for the feedback (as it was me wot writ the Wiki article).

Further to my recent post, I've just had a plough through 17th ed. and have tried to understand what they say about this issue.

As far as I can make out (although I'm very open to correction), and somewhat to my surprise, they seem to be saying that no allowance needs to be made for I2/In for most types of protective device - i.e. it is acceptable for the cable to be sized simply on the basis of the 'rating' (i.e. In) of the protective device (which is what most 'lay' people would probably assume). If my interpretation is correct, then maybe the wiki should say just that - i.e. that Iz should be no less than In, without potentially confusing some people by all the other discussion, interesting though it is, about I1 and I2 etc.

Per the regs, the one exception seems to be if the protective device is a semi-enlclosed (i.e.rewirable) fuse, in which case one has to apply a single correction factor of 0.725; it looks to me as if it a total co-incidence that this figure happens to be half the I2/In ratio for Type B MCBs, but I may be wrong.

What do you think?

Kind Regards, John.

 

[JohnW2]

Can I ask a question or two and put forward a theory because, whilst I agree with JohnW2, I don't know the answer?
[snip]
Can we also deduce that the conductor is actually capable of carrying the current of the I2 value safely and the regulations are merely very conservative in the figures we have to use? What current can a 2.5mm² conductor sustain without melting?
If I have it wrong then what is the reasoning and is JohnW2 correct?

Please see the supplementary message to BAS which I have just posted. To my surprise, it does, indeed, seem that (with the single exception of rewirable fuses) the regs are happy with a cable with a current carrying capacity' only just enough to cope with the 'rating' of the protective device (i.e. 32A for a B32 MCB). As you say, the only way one can really rationalise this is by assuming that they know that the tabulated 'current carrying capacity' is conservative to the extent that a cable can acceptably take (using BAS's figures) 13% more than that indefinitely and 45% more than that for at least an hour.

If that's true, it does nothing to increase my respect for the Wiring Regulations - since they are undermining their own concept of tabulated current carrying capacities - particularly in relation to the 13% 'for ever' potential scenario. In other contexts, I'm sure that those who wrote the regs wouldn't be happy with my using an undersized cable on the basis that the maximum load was 'no more than 13%' (or even 'no more than 45%) above the officially 'permissible' level!

I fear that regulations don't get bad reputations for nothing!

Kind Regards, John

 

[NotHimAgain]

The relationship is clearly stated in BS 7671 - 433.1.1

Ib <= In < = Iz and I2 <= 1.45 Iz

I2 is the current ensuring effective operation in conventional time.

Conventional time is a value stated for each device in its relevant standard. For BS 60898 mcbs conventional time is 1hour for types B, C or D mcbs < = 63A, and 2 hours for > 63A.

The 1.45 factor is similar to a term called Fusing Factor used in the 14th edition of the Wiring Regulations.

In fact each type of device has a different 'fusing factor' - but for BS 88 and BS 1361 fuses, and BS 60898 mcbs the figure of 1.45 is used. BS 3036 (rewireable) fuses have a factor of 2 but it is not expressed like this in BS 7671. See 433.1.3.

The cable current ratings in Appendix 4 take account of the fact that a cable would have to carry I2 for a specified time - so please apologize to the Wiring Regulations .

 

[JohnW2]

The relationship is clearly stated in BS 7671 - 433.1.1
Ib <= In < = Iz and I2 <= 1.45 Iz
I2 is the current ensuring effective operation in conventional time.
Conventional time is a value stated for each device in its relevant standard. For BS 60898 mcbs conventional time is 1hour for types B, C or D mcbs < = 63A, and 2 hours for > 63A.
The 1.45 factor is similar to a term called Fusing Factor used in the 14th edition of the Wiring Regulations.
In fact each type of device has a different 'fusing factor' - but for BS 88 and BS 1361 fuses, and BS 60898 mcbs the figure of 1.45 is used. BS 3036 (rewireable) fuses have a factor of 2 but it is not expressed like this in BS 7671. See 433.1.3.

Thanks. Yes, I understand all that, and don't think any of it is in dispute. However, it is not directly applicable to cable sizing. Indeed, I think that's what makes the wiki a bit confusing, because it also discusses all of the above in relation to cable sizing (determination of required Iz), even though those figures (1.45 aand 2) are not actually relevant to selecting required cables size. If my understanding of the regulations is correct, the required Iz (as tabulated) appears to be simply In in most cases (e.g. where the protective device in an MCB), or 0.725In in the case of a rewirable fuse - but the wiki makes it sound much more complicated and confusing than that.

The wiki also obvioulsy somewhat adds to a reader's confusion by pointing out that the 0.725 'de-rating' factor for a semi-enclosed fuse is half of the 'fuse factor' for an MCB - even though this seems to be nothing but co-incidence!

The cable current ratings in Appendix 4 take account of the fact that a cable would have to carry I2 for a specified time - so please apologize to the Wiring Regulations .

I don't have immediate access to the Appendices, but I have no reason to doubt what you are saying. Do I take it that this is explicitly stated in the Appendix 4? If so, I will partly apologise to the regulations. However, particularly given that In is what 'is written on the tin' I remain unimpressed by any document which writes:
I2 <= 1.45 Iz, where I2=1.45In
... rather than simply In <= Iz !!
(or, even more logical and helpful, since we're given In and are interested in Iz):
Iz > In
Furthermore, if the Iz figures in Appendix 4 actually allows for currents >Iz for up to I hour does this mean more generally that a design could be compliant with the regulations if it was based on an assumption that current might exceed a cable's Iz by up to 45%, provided one knew that this was never going to persist for more than 1 hour. I would suspect not, but where then would be the consistency?

Kind Regards, John

 

[ban-all-sheds]

My concern is that, from considerations of safety (that's surely what current carrying capacity of cables is all about?) one should be dividing, not multiplying, the cable's current carrying capacity by the factor (I2/In), whatever it may be (1.45, 2.0 etc.) for the device concerned (if one wanted to compare the cable's capacity with In) - or, equivalently, multiplying the device's In by that factor in order to determine the cable carrying capacity required.

As skenk says, the actual current that a cable can carry, for shortish-durations, is greater than the tabulated values. It is deemed, by the regulations, to be able to carry 1.45 x Iz for the amount of time it will take the device to operate, and by implication currents > Iz and < 1.45Iz for longer periods than the fusing/tripping time.

Exactly, and that's why I would have assumed that, in the interests of safety, one should use a cable whose capacity is certainly adequate to carry the current that the device could pass 'for ever' and probably also the current which (in the case of an MCB) the device could allow to pass for up to 1 hour? This article is stating (in two different ways) that it is adequate for the cable's tabulated current carrying capacity to only be adeqaute for In, but not for I1 or I2. Whilst I realise that there is undoubtedly a 'safety margin' built into the tabulated 'current carrying capacities' of cables, such a practice would seem to make a mockery of having such tabulated values which we are expected to work to.

It doesn't make a mockery of it - the tabulated values in Appendix 4 allow for I1 & I2 being > Iz (but I2 being &#8804; 1.45Iz)

I don't think I've seen (nor would expect to see) anything to suggest that it's OK to exceed tabulated current carrying capacities by 13% indefinitely and 45% for up to an hour - but maybe I've missed that.

It's not OK for the design current Ib and therefore the device rating In to exceed the tabulated cable capacity Iz because if it does you lose the margins which allow the cable to cope with the fact that I2 > I1 &#8805; Iz.

since you are using the example factor (I2/In) of 1.45, then 1.45Iz>I2 is mathematically exactly the same as In<Iz. The same would be true if you were talking about a device for which the I2/In factor was something other than 1.45. If you call that factor f, then fIz>I2 is always going to be the same as In<Iz.

Yes, it would, but the requirement doesn't change as f changes, i.e. if f was 1.8 then the requirement for Iz remains that 1.45Iz &#8805; I2 (or I2 &#8804; 1.45Iz if you prefer).

It does not become 1.8Iz &#8805; I2.

multiplying by 0.725 is the same as assuming an I2/In ratio of 1.38,

No it's not - it's assuming an I2/In ratio of 2 and adjusting Iz so that I2 remains &#8804; 1.45Iz.

However, if you are saying that the I2/In ratio for a rewirable fuse is 2.0, then I would have thought that one surely should be dividing the cable's tabulated Iz by 2 (rather than 1.38 ) if one is going to compare it with In to determine whether the cable is adequate to carry I2.

No, because the cable's Iz value is chosen such that it will not be damaged by a current of 1.45Iz flowing for a short-ish period.

However, from what you go on to say, it sounds as if the 2.0 ratio may be something more confusing/messy than simply I2/In for a rewirable fuse.

No, that's exactly what the 2.0 figure is.

As for "0.725 = 1.45/2, i.e. 1.45 ÷ the I2:Iz ratio", I find this even more confusing. As you've said, the 1.45 figure is specific to Type B MCBs - so are you saying that all other protective devices are referenced by a 'correction factor/ratio' to Type B MCBs. Whatever, there has surely got to be something wrong with dividing an adjustment factor of 1.45 (an increase of 45%) by 2 and thereby turing it into an adjustment factor of 0.725 (a decrease of 27.5%) would appear rather bizarre. There is surely no sense in the implication that a cable needs 'de-rating' (if one is going to size it in relation to In) in the case of a rewirable fuse (which makes sense) but can be 'up-rated' for an MCB (which does not make sense)!!

No, that's not what's going on, but I can see why you're confused.

Firstly and many apologies - with all the copying and pasting of all the various I<whatever> variables I got one wrong, and I should have written

"0.725 = 1.45/2, i.e. 1.45 ÷ the I2:In ratio"

So that definitely didn't help, but I still think you are also getting confused about the 1.45 value.

The 1.45 in "0.725 = 1.45/2" is not the ratio of I2:In for a Type B breaker it is the 1.45 in 1.45Iz &#8805; I2.

The 2 in "0.725 = 1.45/2" is the I2:In for a 3036 fuse.


I was also wrong, in practice, when I said that "for any other device you should do a second check, in addition to Ib &#8804; In &#8804; Iz for 1.45Iz&#8805;I2 when choosing a cable".

In theory you should, I guess, but in practice the regs say that for all other protective devices you can ignore the fusing factor (I2:In) and simply ensure that Iz &#8805; In.

Were it not for that rule of thumb then I think it would all be a lot clearer.

The table below shows the fusing factor for various devices, and in the last column is the value by which the cable capacity would have to be multiplied by if one wanted a strict non-rule-of-thumb adherence to 1.45Iz&#8805;I2:

[code:1] Type I2:I1 1.45 ÷ I2:I1

BS 88 fuse 1.6 0.906

BS 1361 fuse 1.5 0.967

BS 1362 fuse 1.9 0.763

Type 1/2/3/4 MCB up to 10A 1.5 0.967

Type 1/2/3/4 MCB over 10A 1.35 1.074 **

Type B/C/D MCB 1.45 1.0

BS 3036 fuse 2.0 0.725
[/code:1]

** An interesting value - in reality if you were doing this you'd have to limit it to 1 to ensure In &#8804; Iz

As you can see they are all close enough to 1 for it to make sense that only for 3036 and 1362 fuses do you need to bother adjusting Iz so that I2 remains &#8804; 1.45Iz.

This really does seem all wrong to me. I would have expected that the regulations would be simple, merely requiring that cables should be sized so as to have an adequate current carrying capacity to carry that current which the protective device colud allow to flow for appreciable periods of time (i.e. certainly I1 and probably I2)

They are and they do - the tabulated values for current carrying capacity allow for this:

As far as I can make out (although I'm very open to correction), and somewhat to my surprise, they seem to be saying that no allowance needs to be made for I2/In for most types of protective device - i.e. it is acceptable for the cable to be sized simply on the basis of the 'rating' (i.e. In) of the protective device (which is what most 'lay' people would probably assume).

That's because for most types of protective device the value of 1.45 ÷ the device's fusing factor, I2/In is close enough to 1 for no allowance to be needed.

If my interpretation is correct, then maybe the wiki should say just that - i.e. that Iz should be no less than In, without potentially confusing some people by all the other discussion, interesting though it is, about I1 and I2 etc.

I think it's worthwhile to keep the 1.45Iz &#8805; I2 in there, but to add more information on what I2 is, and how I2:In is close enough to 1.45 for most protective devices for it to be disregarded, and only for BS 3036 and 1362 fuses does it need to be considered.

I should probably also add an explanation of just what the purpose of the adjustment is to make sure people have got it the right way round, i.e. if you've got a 30A rewirable fuse it's not that you're looking for a cable with a capacity of 21.75A, you're looking for a cable with a capacity which when multiplied by 0.725 is no less than 30A.

Per the regs, the one exception seems to be if the protective device is a semi-enlclosed (i.e.rewirable) fuse, in which case one has to apply a single correction factor of 0.725; it looks to me as if it a total co-incidence that this figure happens to be half the I2/In ratio for Type B MCBs, but I may be wrong.

What do you think?

TBH I'm not sure how much of a coincidence it is.

It's half of something because the fusing current for a 3036 is 2 x its nominal rating.

It's half of a value which is the I2:In for a type B and which is also the required maximum ratio of I2:Iz.

The Q is is it a coincidence that the maximum ratio is the same as the fusing factor for a Type B, or was the value chosen to be the fusing factor?

Or, was that value of 1.45 already established as the maximum safe value and BS EN 60898 written to specify that the fusing factor for devices was to be no more than 1.45?

If that's true, it does nothing to increase my respect for the Wiring Regulations

Why?

since they are undermining their own concept of tabulated current carrying capacities - particularly in relation to the 13% 'for ever' potential scenario. In other contexts, I'm sure that those who wrote the regs wouldn't be happy with my using an undersized cable on the basis that the maximum load was 'no more than 13%' (or even 'no more than 45%) above the officially 'permissible' level!

No, they wouldn't be happy, because that's not what they are doing.

The tabulated values for current capacity are not what the cable can carry before becoming damaged. They are the values which, after correction for things like installation method, grouping, ambient temperature, type of protective device etc, give you an Iz which is safe to use when what's going on is as described pictorially in the diagram above.

Indeed, I think that's what makes the wiki a bit confusing, because it also discusses all of the above in relation to cable sizing (determination of required Iz), even though those figures (1.45 aand 2) are not actually relevant to selecting required cables size.

Actually they are - the problem with the Wiki is not that it contains too much information it's that it doesn't have enough. If (when) it's expanded to cover fusing factors then the relevance of 1.45Iz should become clear.

If my understanding of the regulations is correct, the required Iz (as tabulated) appears to be simply In in most cases (e.g. where the protective device in an MCB), or 0.725In in the case of a rewirable fuse

No!

The required Iz is not 0.725In in the case of a rewirable fuse, you've got that the wrong way around.

If you have a rewirable fuse then the cable's Iz value has to be multiplied by 0.725. i.e. if you are trying to determine what the Iz of the cable is, you start with its tabulated value, you apply any de-rating factors for grouping, ambient temperature, thermal insulation etc and the de-rating factor (i.e. x) of 0.725. If the value which falls out the end is < In you're hosed.

If you're trying to find out what the required value of Iz is when using a rewirable, you divide the fuse rating by 0.725. e.g. for a 30A fuse you need your cable, after you've applied all the other de-rating factors to it as normal to have a rating of 41.4A.

but the wiki makes it sound much more complicated and confusing than that.

Hmm.

I didn't think so, but after this....

The wiki also obvioulsy somewhat adds to a reader's confusion by pointing out that the 0.725 'de-rating' factor for a semi-enclosed fuse is half of the 'fuse factor' for an MCB - even though this seems to be nothing but co-incidence!

Oh be fair - it doesn't mention any of that at all.

I don't have immediate access to the Appendices, but I have no reason to doubt what you are saying. Do I take it that this is explicitly stated in the Appendix 4?

No, it isn't, but it doesn't need to be as for protective devices it doesn't talk about I1 or I2, only In.

However, particularly given that In is what 'is written on the tin' I remain unimpressed by any document which writes:
I2 <= 1.45 Iz, where I2=1.45In
... rather than simply In <= Iz !!

As explained above, those two 1.45s are not the same thing. They are two different things which happen to have the same value, and the simplification you seek only works when they do. It's possible for one of them to change (e.g. to 2.0 for 3036 fuses), but the other one remains at 1.45.

Even if the 1.45s are the same because one was chosen to be what the other one is (i.e. the fusing factor for Type Bs) or because the fusing factor for Type Bs was written into the spec to be what was known to be the safe value for cables (i.e. 1.45) that does not alter the fact that they are not the same thing, they just have the same value.

(or, even more logical and helpful, since we're given In and are interested in Iz):
Iz > In

But to calculate Iz you have to allow for the fact that I2 is not always 1.45In, but must always be &#8804; 1.45Iz....

Furthermore, if the Iz figures in Appendix 4 actually allows for currents >Iz for up to I hour does this mean more generally that a design could be compliant with the regulations if it was based on an assumption that current might exceed a cable's Iz by up to 45%, provided one knew that this was never going to persist for more than 1 hour. I would suspect not, but where then would be the consistency?

Well, ignoring the provisions for cyclic loads and thermally equivalent constants you suspect right, and the consistency you seek lies in the diagram above - all the cable ratings are based on the assumption that Iz lies between In & I1 and that I2 &#8804; 1.45Iz....

 

[NotHimAgain]

The notes to Appendix 4 detail the rational behind the various factors (see section 4) - further information is available in GN 6 Protection Against Over-Current, and various Commentaries on the Wiring Regulations.

Appendix 4 is mainly the work of the organisation that was the Electrical Research Association (ERA) - it is now a private company. Appendix 4 is considered to be one of the best sources of cable current rating available anywhere in the world.

However, it is just an 'informative' appendices and as such entirely optional.

If you want to go it alone you will need the BS IEC 60287 series of standards and probably at least a first degree in electrical engineering .

 

[1john]

Bloody hell BAS, top marks, somethimes I think you are the regs (all of them, in the world, ever....)

 

[JohnW2]

BAS, thanks for your very detailed response, which is very helpful. I now fully understand what the regs are saying, but I think this process has underlined the desirability of making the wiki a lot clearer (and maybe simpler, although I know you seem to disagree with that). Like you, I also have one typo to apologise for (see below). Anyway, the crux of my confusion related to:

As skenk says, the actual current that a cable can carry, for shortish-durations, is greater than the tabulated values. It is deemed, by the regulations, to be able to carry 1.45 x Iz for the amount of time it will take the device to operate, and by implication currents > Iz and < 1.45Iz for longer periods than the fusing/tripping time.

This is what I hadn't realised. If you look at the wiki, I don't think you can blame me for having thought that the 1.45 (all the 1.45s !) related only to the I2/In ratio for a Type B MCB. As you now explain, the 1.45 factor is the deemed 'temporary overload' capacity characteristic of a cable, and the I2/In ratio for a Type B MCB is also 1.45 because the 'fusing factor multiplication factor' is exactly 1.0 for such a device. Now that I understand that, most of my confusion has gone - but I do think that the wording of the wiki is what started me down my wrong path. As you later write yourself:

The 1.45 in "0.725 = 1.45/2" is not the ratio of I2:In for a Type B breaker it is the 1.45 in 1.45Iz &#8805; I2.

... and I think that explains/excuses the confusion I had after reading the wiki!

.... but in practice the regs say that for all other protective devices [other than BS3036/1362 fuses] you can ignore the fusing factor (I2:In) and simply ensure that Iz &#8805; In.
Were it not for that rule of thumb then I think it would all be a lot clearer.

I suppose that's one way of looking at it - but, as I went on to say, given the likely readers of the wiki, and given that the wiki is specifically about Type B MCBs, I would be more inclined to only mention the 'rule of thumb'. Indeed, in the case of a Type B MCB, it's not a rule-of-thumb at all, but actually precisely correct, since the fusing factor correction is exactly 1.0. As you say:

As you can see they are all close enough to 1 for it to make sense that only for 3036 and 1362 fuses do you need to bother adjusting Iz so that I2 remains &#8804; 1.45Iz.

and

That's because for most types of protective device the value of 1.45 ÷ the device's fusing factor, I2/In is close enough to 1 for no allowance to be needed.

Per the regs, the one exception seems to be if the protective device is a semi-enlclosed (i.e.rewirable) fuse, in which case one has to apply a single correction factor of 0.725; it looks to me as if it a total co-incidence that this figure happens to be half the I2/In ratio for Type B MCBs, but I may be wrong.

What do you think?

TBH I'm not sure how much of a coincidence it is.

As at the start, now that I understand what's going on (i.e that the fundamental 1.45 is a deemed characteristic of the cable), it is clear that it is not a co-incidence at all.

If my understanding of the regulations is correct, the required Iz (as tabulated) appears to be simply In in most cases (e.g. where the protective device in an MCB), or 0.725In in the case of a rewirable fuse

No!
The required Iz is not 0.725In in the case of a rewirable fuse, you've got that the wrong way around.

Mea culpa - that was my typo. It should have read "In/0.725". However, with that correction, I think that sentence is what (probably all) a reader of the wiki reallky needs to know, and anything more one tells them is more likley to confuse than assist.

(or, even more logical and helpful, since we're given In and are interested in Iz):
Iz > In

But to calculate Iz you have to allow for the fact that I2 is not always 1.45In, but must always be &#8804; 1.45Iz....

Agreed, but I was talking in relation to the wiki, which specifically says that it's talking only about Type B MCBs. This is what most readers will be interested in, and the wiki already says that figures for other protective devices will not be the same.

Kind Regards, John

 

[JohnW2]

Per the regs, the one exception seems to be if the protective device is a semi-enlclosed (i.e.rewirable) fuse, in which case one has to apply a single correction factor of 0.725; it looks to me as if it a total co-incidence that this figure happens to be half the I2/In ratio for Type B MCBs, but I may be wrong.
What do you think?

TBH I'm not sure how much of a coincidence it is.

As at the start, now that I understand what's going on (i.e that the fundamental 1.45 is a deemed characteristic of the cable), it is clear that it is not a co-incidence at all.

On looking back at this, the point I should have added is that the thing which is a total co-incidence, and which facilitated all the confusion I suffered, is the fact that the degree of temporary overload a cable is deemed to be able to cope with (factor of 1.45) is the same as the fusing factor (I2/In) of a Type B MCB (also 1.45). Admittedly, the fact that they are exactly the same may be the result of a slight tweak of the 'temporary overload factor', so as to make it identical to the MCB's fusing factor - but it would still be a total co-incidence that it was close enough the the MCB's fusing factor for such a tweak to have been reasonable.

Kind Regards, John

 

[EFLImpudence]

In view of the fact that cable has been around longer than mcbs, could it be that mcbs, when invented, had to be manufactured to these values to be able to be incorporated into an installation.

So it was the mcb that was matched to the cable even though now, apparently, we seem to be selecting cable to match the mcb.